Amino Acid Metabolism Practice Questions

Q: What is the primary role of glutamine in blood transport?

Ammonia transport. **Explanation:** **1. Why Ammonia Transport is Correct:** Ammonia ($NH_3$) is a highly toxic byproduct of protein catabolism, especially in peripheral tissues like skeletal muscle and the brain. To prevent neurotoxicity, ammonia must be transported to the liver for detoxification into urea. Glutamine serves as the **primary non-toxic transport form of ammonia** in the blood. The enzyme **Glutamine Synthetase** catalyzes the addition of ammonia to glutamate to form glutamine. Once it reaches the liver or kidneys, the enzyme **Glutaminase** cleaves it back into glutamate and free ammonia, which then enters the Urea Cycle. **2. Why Other Options are Incorrect:** * **Toxic element:** Glutamine is a physiological, non-toxic amino acid. In fact, its formation is a key **detoxification mechanism** to remove toxic free ammonia from circulation. * **Energy storage:** While glutamine can be oxidized for energy (especially by enterocytes and lymphocytes), its *primary* role in systemic blood transport is nitrogen/ammonia movement, not as a major energy reservoir like glycogen or triglycerides. * **Abnormal metabolite:** Glutamine is the most abundant free amino acid in the human body and is a normal, essential component of protein metabolism. **Clinical Pearls for NEET-PG:** * **Brain Protection:** In the brain, glutamine synthesis is the major mechanism for removing ammonia. However, excessive glutamine accumulation in astrocytes leads to osmotic swelling, contributing to **Hepatic Encephalopathy**. * **Renal Role:** In the kidneys, glutamine metabolism provides ammonia which acts as a buffer ($NH_3 + H^+ \rightarrow NH_4^+$) to excrete protons during **metabolic acidosis**. * **Alanine vs. Glutamine:** Remember that **Alanine** is the primary transporter of ammonia from **Skeletal Muscle** (Glucose-Alanine Cycle), while **Glutamine** is the universal transporter from most other tissues, including the brain.

Q: Which of the following is a mitochondrial enzyme of the Urea Cycle?

Carbamoyl Phosphate synthetase I. The Urea Cycle (Krebs-Henseleit cycle) is a metabolic pathway that occurs exclusively in the liver to detoxify ammonia. A key high-yield concept for NEET-PG is the **compartmentalization** of its enzymes between the mitochondria and the cytosol. ### 1. Why Carbamoyl Phosphate Synthetase I (CPS-I) is Correct The first two steps of the urea cycle occur within the **mitochondrial matrix**. * **CPS-I** is the rate-limiting enzyme that catalyzes the condensation of $NH_4^+$ and $HCO_3^-$ to form Carbamoyl Phosphate. * It requires **N-acetylglutamate (NAG)** as an essential allosteric activator. * *Note:* The only other mitochondrial enzyme in this cycle is **Ornithine Transcarbamoylase (OTC)**. ### 2. Analysis of Incorrect Options * **Arginase (Option A):** This is the final enzyme of the cycle, located in the **cytosol**. It cleaves Arginine into Urea and Ornithine. * **Arginosuccinate Lyase (Option C):** Located in the **cytosol**, it cleaves arginosuccinate into arginine and fumarate (linking the urea cycle to the TCA cycle via the "aspartate-arginosuccinate shunt"). * **Arginosuccinate Synthetase (Option D):** Located in the **cytosol**, it condenses citrulline with aspartate. ### 3. High-Yield Clinical Pearls * **Mnemonic:** Remember "**M**other **O**nly" for Mitochondrial enzymes (**M**atrix): **M**-CPS-I and **O**-OTC. * **Rate-Limiting Step:** CPS-I is the rate-limiting enzyme of the urea cycle. * **Most Common Deficiency:** **OTC deficiency** is the most common urea cycle disorder and is the only one that is **X-linked recessive** (others are autosomal recessive). * **Hyperammonemia:** Defects in any of these enzymes lead to ammonia toxicity, presenting clinically with lethargy, vomiting, and cerebral edema.

Q: Gamma-aminobutyric acid (GABA) is synthesized from which precursor molecule?

Glutamate. ### Explanation **Correct Answer: B. Glutamate** **Mechanism:** Gamma-aminobutyric acid (GABA), the primary inhibitory neurotransmitter in the central nervous system, is synthesized from **L-glutamate**. This conversion is catalyzed by the enzyme **Glutamic Acid Decarboxylase (GAD)**. This reaction is a decarboxylation process that requires **Pyridoxal Phosphate (Vitamin B6)** as a mandatory cofactor. **Analysis of Incorrect Options:** * **A. Fumarate:** This is an intermediate of the TCA cycle and the urea cycle. While it can be used to regenerate aspartate, it is not a direct precursor to GABA. * **C. Histidine:** This amino acid is the precursor for **Histamine**, catalyzed by histidine decarboxylase (also B6 dependent). * **D. Glycine:** While glycine itself acts as an inhibitory neurotransmitter (primarily in the spinal cord), it is not a precursor for GABA. It is involved in the synthesis of heme, purines, and creatine. **Clinical Pearls for NEET-PG:** 1. **Vitamin B6 Deficiency:** Since GAD requires PLP (B6), a deficiency in Vitamin B6 leads to decreased GABA levels. This results in CNS over-excitation, manifesting as **convulsions** (commonly seen in infants). 2. **GABA Shunt:** This is a bypass of the TCA cycle where Glutamate → GABA → Succinic Semialdehyde → **Succinate**. This allows the brain to utilize GABA as an energy source during metabolic stress. 3. **Inhibitory Action:** GABA acts by increasing chloride conductance through GABA-A receptors, leading to hyperpolarization of the postsynaptic neuron.

Q: Tryptophan is glucogenic and ketogenic by producing which of the following?

Acetyl CoA and alanine. **Explanation:** Tryptophan is one of the few amino acids that is both **glucogenic and ketogenic**. Its complex catabolic pathway (the Kynurenine-Anthranilate pathway) leads to the cleavage of the molecule into two distinct fragments: 1. **Ketogenic component:** Tryptophan is converted into **Acetoacetyl-CoA**, which then forms **Acetyl-CoA**. This enters the ketogenic pool. 2. **Glucogenic component:** During the pathway, the side chain of tryptophan is released as **Alanine**. Alanine is a premier glucogenic amino acid as it can be transaminated to pyruvate, a direct precursor for gluconeogenesis. **Analysis of Incorrect Options:** * **Option B (Acetoacetate and Fumarate):** This describes the catabolism of **Phenylalanine and Tyrosine**. While they are also both glucogenic and ketogenic, they yield fumarate (glucogenic) and acetoacetate (ketogenic). * **Option C & D (Arginine):** Arginine is a purely glucogenic amino acid that enters the TCA cycle via α-ketoglutarate. It is not a product of tryptophan catabolism. **High-Yield NEET-PG Pearls:** * **Purely Ketogenic Amino Acids:** Leucine and Lysine (The "L"s). * **Both Glucogenic & Ketogenic:** Phenylalanine, Tyrosine, Tryptophan, and Isoleucine (Mnemonic: **P**hil **T**ries **T**o **I**solate). * **Clinical Correlation:** Tryptophan is the precursor for **Niacin (Vitamin B3)**, **Serotonin**, and **Melatonin**. * **Hartnup Disease:** A defect in the transport of neutral amino acids (like Tryptophan) leading to Pellagra-like symptoms because Tryptophan is unavailable for Niacin synthesis.

Q: Which amino acid is regarded as the collector of amino groups?

Glutamate. **Explanation:** **Glutamate** is known as the "collector of amino groups" because of its central role in **transamination** and **deamination** reactions. During protein catabolism, most amino acids transfer their $\alpha$-amino group to $\alpha$-ketoglutarate (via aminotransferases), resulting in the formation of Glutamate. This process effectively funnels nitrogen from various amino acids into a single pool. Glutamate then undergoes **oxidative deamination** by the enzyme *Glutamate Dehydrogenase* in the mitochondria, releasing the amino group as free ammonia ($NH_3$) for the Urea Cycle. **Analysis of Incorrect Options:** * **Aspartate:** While it is an important nitrogen donor in the Urea Cycle (providing the second nitrogen atom to form Argininosuccinate), it is not the primary collector from all other amino acids. * **Arginine:** This is a semi-essential amino acid and an intermediate of the Urea Cycle. Its primary role is as a precursor for Nitric Oxide (NO) and Urea, not as a nitrogen collector. * **Cysteine:** This is a sulfur-containing amino acid involved in glutathione synthesis and disulfide bond formation; it does not play a central role in general amino group collection. **High-Yield Facts for NEET-PG:** * **Glutamine** is the major transport form of ammonia from peripheral tissues (like muscle and brain) to the liver. * **Alanine** is the primary transporter of amino groups from the muscle to the liver (Glucose-Alanine Cycle). * **Glutamate Dehydrogenase** is the only enzyme that can use either $NAD^+$ or $NADP^+$ as a coenzyme. * **$\alpha$-Ketoglutarate** is the universal acceptor of amino groups during transamination.

Q: A 2-month-old infant presented with hepatic crisis, peripheral neuropathy, and Fanconi-like syndrome. Laboratory investigations showed elevated transaminases, low coagulation factors, and elevated succinylacetone in serum and urine. What is the most likely diagnosis?

Tyrosinemia. **Explanation:** The clinical presentation and laboratory findings point directly to **Tyrosinemia Type I** (Hepatorenal Tyrosinemia). **Why Tyrosinemia Type I is correct:** Tyrosinemia Type I is caused by a deficiency of the enzyme **Fumarylacetoacetate Hydrolase (FAH)**, the final step in the tyrosine degradation pathway. This deficiency leads to the accumulation of fumarylacetoacetate, which is diverted into the formation of **Succinylacetone**. * **Succinylacetone** is the pathognomonic marker for this condition; it is toxic to the liver (causing cirrhosis and hepatic crisis) and the kidneys (causing **Fanconi-like syndrome** with renal tubular acidosis). * It also inhibits heme synthesis, leading to porphyria-like neurological crises (**peripheral neuropathy**). **Why other options are incorrect:** * **Phenylketonuria (PKU):** Caused by Phenylalanine Hydroxylase deficiency. Presents with intellectual disability, "mousy" odor, and hypopigmentation, but not hepatic failure or succinylacetone elevation. * **Homocystinuria:** Caused by Cystathionine $\beta$-synthase deficiency. Characterized by ectopia lentis, marfanoid habitus, and thromboembolism. * **Hawkinsinuria:** A rare defect in the 4-hydroxyphenylpyruvate dioxygenase enzyme. It presents with failure to thrive and metabolic acidosis in infancy but lacks the severe hepatic crisis and succinylacetone marker. **NEET-PG High-Yield Pearls:** * **Diagnostic Marker:** Succinylacetone in urine/blood (Most specific). * **Treatment of Choice:** **Nitisinone (NTBC)**, which inhibits 4-hydroxyphenylpyruvate dioxygenase, preventing the formation of toxic metabolites. * **Long-term Risk:** Patients have a very high risk of developing **Hepatocellular Carcinoma (HCC)**. * **Dietary Management:** Restriction of Phenylalanine and Tyrosine.

Q: A 30-year-old chronically malnourished male presented with weakness, lassitude, anorexia, poor appetite, abdominal discomfort, and diarrhea. He also exhibits apathy, lethargy, and mild disorientation. On examination, glossitis and dermatitis in sun-exposed areas are noted. What is the chromosome on which the gene for the metabolic disorder that mimics these findings is present?

5p. **Explanation:** The clinical presentation of dermatitis (in sun-exposed areas), diarrhea, and dementia (apathy/disorientation), combined with glossitis, is the classic triad of **Pellagra**, caused by a deficiency of **Niacin (Vitamin B3)**. The question asks for the chromosome of a metabolic disorder that *mimics* these findings. This refers to **Hartnup disease**, an autosomal recessive disorder characterized by a defect in the transport of neutral amino acids (including **Tryptophan**) in the renal tubules and intestines. Since Tryptophan is a precursor for Niacin synthesis (60 mg Tryptophan = 1 mg Niacin), its deficiency leads to pellagra-like symptoms. The gene responsible for Hartnup disease is **SLC6A19**, which is located on **Chromosome 5p**. **Analysis of Options:** * **A. 5p (Correct):** Location of the SLC6A19 gene. Mutations here cause Hartnup disease, mimicking Niacin deficiency. * **B. 21p:** Associated with Down Syndrome (Trisomy 21) and certain amyloid proteins, but not neutral amino acid transport. * **C. 6p:** Contains the HLA complex (Major Histocompatibility Complex), linked to various autoimmune diseases but not pellagra-like symptoms. * **D. 8p:** Associated with conditions like Spherocytosis or certain neurodevelopmental disorders, but irrelevant to Tryptophan metabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Pellagra 4 D’s:** Dermatitis (Casal’s necklace), Diarrhea, Dementia, and Death. * **Hartnup Disease Diagnosis:** High levels of neutral amino acids in urine (**Neutral Aminoaciduria**) but normal levels in blood. * **Niacin Synthesis:** Requires **Vitamin B6 (Pyridoxine)** as a cofactor (Kynureninase enzyme). Deficiency of B6 can also cause secondary pellagra. * **Dietary Link:** Pellagra is common in populations consuming **Maize (Corn)** as a staple, as the niacin in maize is bound (niacytin) and it is deficient in Tryptophan.

Q: A 15-year-old girl is seen by a dermatologist for removal of multiple squamous cell carcinomas of the skin. The patient has nearly white hair, pink irises, very pale skin, and a history of burning easily when exposed to the sun. This patient's condition is caused by a disorder involving which of the following substances?

Aromatic amino acids. ### Explanation The clinical presentation of very pale skin, white hair, pink irises (transillumination), and early-onset squamous cell carcinomas (SCC) is diagnostic of **Oculocutaneous Albinism (OCA)**. **1. Why Aromatic Amino Acids is Correct:** OCA is most commonly caused by a deficiency or defect in the enzyme **tyrosinase**. Tyrosinase is the rate-limiting enzyme in melanogenesis, responsible for converting **Tyrosine** into DOPA and subsequently into melanin. Tyrosine is an **aromatic amino acid**. Without functional tyrosinase, melanocytes cannot produce melanin, leading to a total lack of pigment in the skin, hair, and eyes. This lack of protective melanin results in extreme photosensitivity and a high risk of UV-induced skin cancers (SCC and BCC) at a young age. **2. Why Incorrect Options are Wrong:** * **B. Branched-chain amino acids (Leucine, Isoleucine, Valine):** Defects here lead to **Maple Syrup Urine Disease (MSUD)**, characterized by neurological deterioration and a burnt-sugar odor in urine, not pigmentary disorders. * **C. Glycolipids:** Disorders of glycolipid metabolism (e.g., Gaucher or Fabry disease) are **Lysosomal Storage Diseases**, typically presenting with organomegaly and neurological or renal issues. * **D. Glycoproteins:** Defects in glycoprotein metabolism or transport (e.g., I-cell disease) lead to skeletal abnormalities and developmental delays. **3. NEET-PG High-Yield Pearls:** * **Inheritance:** Most forms of OCA are **Autosomal Recessive**. * **Melanocyte Count:** In Albinism, the **number of melanocytes is normal**, but melanin production is absent. (Contrast this with Vitiligo, where melanocytes are destroyed). * **Visual Acuity:** Patients often have nystagmus and reduced visual acuity due to foveal hypoplasia. * **Chediak-Higashi Syndrome:** A related condition featuring partial albinism, giant lysosomal granules, and recurrent infections.

Q: With ageing, a slight decrease in cognitive impairment is seen due to an increase in the level of which of the following?

Homocysteine. **Explanation:** **Correct Option: C. Homocysteine** The correct answer is **Homocysteine**. Elevated levels of homocysteine in the blood (**Hyperhomocysteinemia**) are strongly associated with cognitive decline, dementia, and Alzheimer’s disease in the elderly. Homocysteine is a sulfur-containing intermediate in methionine metabolism. It exerts neurotoxic effects through several mechanisms: 1. **Vascular Damage:** It promotes oxidative stress and endothelial dysfunction, leading to cerebral microvascular disease. 2. **Direct Neurotoxicity:** It acts as an agonist at NMDA receptors, leading to calcium influx and neuronal apoptosis. 3. **DNA Damage:** It interferes with methylation reactions essential for DNA repair in neurons. **Why other options are incorrect:** * **A. Methionine:** This is an essential amino acid and the precursor to homocysteine. While it is vital for protein synthesis, its direct elevation is not a primary biomarker for age-related cognitive impairment. * **B. Cysteine:** Homocysteine is converted to cysteine via the cystathionine $\beta$-synthase (CBS) pathway (transsulfuration). Cysteine is a precursor for glutathione (an antioxidant) and is generally considered neuroprotective rather than a cause of cognitive decline. * **D. Taurine:** Derived from cysteine, taurine acts as an inhibitory neurotransmitter and antioxidant. It is often studied for its potential neuroprotective benefits in aging. **NEET-PG High-Yield Pearls:** * **The Triple Deficiency:** Hyperhomocysteinemia is often caused by deficiencies in **Vitamin B12 (Cobalamin), B9 (Folate), and B6 (Pyridoxine)**. * **Key Enzymes:** Methionine Synthase (requires B12 and Folate) and Cystathionine $\beta$-synthase (requires B6). * **Clinical Link:** High homocysteine is also a significant independent risk factor for **atherosclerosis and coronary artery disease (CAD)**.

Question 1

What is the primary role of glutamine in blood transport?

Accepted Answer

Ammonia transport

Answer

Toxic element

Answer

Energy storage

Answer

Abnormal metabolite

Question 2

Which of the following is a mitochondrial enzyme of the Urea Cycle?

Accepted Answer

Carbamoyl Phosphate synthetase I

Answer

Arginase

Answer

Arginosuccinate lyase

Answer

Arginosuccinate synthetase

Question 3

Gamma-aminobutyric acid (GABA) is synthesized from which precursor molecule?

Accepted Answer

Glutamate

Answer

Fumarate

Answer

Histidine

Answer

Glycine

Question 4

Tryptophan is glucogenic and ketogenic by producing which of the following?

Accepted Answer

Acetyl CoA and alanine

Answer

Acetoacetate and fumarate

Answer

Acetoacetate and arginine

Answer

Arginine and alanine

Question 5

Which amino acid is regarded as the collector of amino groups?

Accepted Answer

Glutamate

Answer

Aspartate

Answer

Arginine

Answer

Cysteine

Question 6

A 2-month-old infant presented with hepatic crisis, peripheral neuropathy, and Fanconi-like syndrome. Laboratory investigations showed elevated transaminases, low coagulation factors, and elevated succinylacetone in serum and urine. What is the most likely diagnosis?

Accepted Answer

Tyrosinemia

Answer

Phenylketonuria

Answer

Homocystinuria

Answer

Hawkinsinuria

Question 7

All of the following may be raised in carcinoid syndrome, except?

Accepted Answer

VMA

Answer

5HIAA

Answer

5HT

Answer

5HTP

Question 8

A 30-year-old chronically malnourished male presented with weakness, lassitude, anorexia, poor appetite, abdominal discomfort, and diarrhea. He also exhibits apathy, lethargy, and mild disorientation. On examination, glossitis and dermatitis in sun-exposed areas are noted. What is the chromosome on which the gene for the metabolic disorder that mimics these findings is present?

Accepted Answer

5p

Answer

21p

Answer

6p

Answer

8p

Question 9

A 15-year-old girl is seen by a dermatologist for removal of multiple squamous cell carcinomas of the skin. The patient has nearly white hair, pink irises, very pale skin, and a history of burning easily when exposed to the sun. This patient's condition is caused by a disorder involving which of the following substances?

Accepted Answer

Aromatic amino acids

Answer

Branched chain amino acids

Answer

Glycolipids

Answer

Glycoproteins

Question 10

With ageing, a slight decrease in cognitive impairment is seen due to an increase in the level of which of the following?

Accepted Answer

Homocysteine

Answer

Methionine

Answer

Cysteine

Answer

Taurine

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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